When a strip of sheet material, such as metal, web or film, must be reduced in gauge or thickness, the material is normally processed by a rolling mill. The rolling mill passes the material strip between rolling cylindrical surfaces under pressure. Ideally, the rolling mill produces a coil of sheet at a thinner and constant gauge.
A typical single stand rolling mill feeds the material from an unwind reel to a rewind reel. The material strip is passed between work rolls that are acted upon by backup rolls. A force is applied to at least one of the backup rolls. Typically, the work rolls, backup rolls and unwind and rewind reels are not perfectly cylindrical due to a number of reasons including temperature effects, wear, and mechanical inconsistencies. As a result, periodic deviations are impressed on the metal throughout the course of rolling. In addition, cyclical perturbations may have been impressed on the material sheets due to earlier processing. Periodic or cyclical perturbations in the gauge are obviously undesirable. Indeed, they constitute the most significant component of material thickness deviation in modern rolling mills. These thickness deviations are an important concern in the production of sheet, web, and film materials such as paper, plastic and metal.
The frequencies of the cyclical perturbations are a function of the rotation frequencies and associated harmonics of the mechanical components used in the rolling process. Typically, the disturbances vary at high frequencies relative to the frequency response of conventional gauge controls. This makes it difficult to eliminate their effects with standard control techniques employing measured thickness. One known solution samples exit thickness of the sheet at fixed angular intervals based on a signal read from an encoder mounted on the roll's axis. This technique synchronizes sampling to the angular displacement of the roll, insuring that the phase relationship between disturbance, measurement, and controls remains fixed. For each rotation of the roll, the fast Fourier transform of the sample signal is computed to determine the phase and magnitude of gauge disturbances at frequencies which are integral multiples of the roll revolution rate. These frequencies are set and only these frequencies are tracked. From this information, an output to control the roll separating force or work roll gap is computed to correct the eccentricity disturbance. However, such a solution requires the mounting and continued adjustment of encoders on the rolls. These systems also ignore cyclical perturbations of incoming material. Also, this solution does not correct for slippage or extrusion occurring in the process which then results in phase shifts. This can cause the rolling mill to actually add to the problem, rather than correct the problem.
The present invention is intended to solve one or more of the problems discussed above in a novel and simple manner.